An image forming apparatus employing electrophotographic process forms electric charge uniformly on an image carrier made of a photoreceptor composed of an inorganic or organic photoconductive material, forms an electrostatic latent image when irradiated with laser obtained by modifying image signal or the like, and then develops the electrostatic latent image with a charged toner to give a visible toner image. The toner image thus obtained is then transferred to a recording medium such as paper directly or via an intermediate transfer medium to obtain a desired reproduced image.
An image forming apparatus employing a process which comprises primarily transferring a toner image formed on an image carrier to an intermediate transfer medium, and then secondarily transferring the toner image from the intermediate transfer medium to a recording medium is disclosed in, e.g., JP-A-62-206567 (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
As the belt material to be incorporated in an image forming apparatus employing an intermediate transfer medium process there has been proposed an electrically-conductive endless belt comprising a thermoplastic resin such as polyvinylidene fluoride (PVDF) (JP-A-5-200904, JP-A-6-228335), polycarbonate (PC) (JP-A-6-95521), polyalkylene terephthalate (PAT) (JP-A-6-149081), blend of PAT and PC (JP-A-6-149083), blend of ethylene-tetrafluoroethylene copolymer (ETFE) and PC, blend of ETFE and PAT and blend of ETFE, PC and PAT (JP-A-6-149079) having an electrically conducting material such as carbon black dispersed therein.
The foregoing electrically-conductive material comprising a thermoplastic resin such as PVDF and PC exhibits mechanical properties as poor as not more than 24,000 kg/cm.sup.2 as determined in terms of Young's modulus. Thus, the belt made of such an electrically-conductive material deforms greatly when stressed during driving. If this belt material is used as an intermediate transfer belt, a high quality transfer image cannot be stably obtained. Further, since the belt is liable to cracking at the edge thereof during driving, it exhibits a poor durability.
One of materials having excellent mechanical properties is a thermosetting polyimide resin. For example, JP-A-63-311263 proposes a seamless belt made of a polyimide resin comprising carbon black dispersed therein. This seamless belt is prepared by a process which comprises dispersing carbon black as an electrically conducting material in a solution of a polyamidic acid as a polyimide precursor, casting the dispersion over a metal drum, drying the material, peeling the film off the metal drum, orienting the film at a high temperature to form a polyimide film, cutting the polyimide film into a proper size, and then forming the film into an endless belt.
An ordinary process for the formation of the foregoing film comprises injecting a polymer solution having carbon black dispersed therein into a cylindrical mold, and then subjecting the polymer solution to centrifugal forming while being rotated at 1,000 to 2,000 rpm and heated to a temperature of from 110.degree. C. to 150.degree. C. so that it is formed into film. The film thus obtained is released half-hardened from the mold, and then put on an iron core where it is then allowed to undergo imidization reaction (ring closure reaction of polyamidic acid) at a temperature of from 300.degree. C. to 450.degree. C. so that it is thoroughly hardened.
In the foregoing rotary forming process such as centrifugal forming, however, if the solvent evaporates unevenly at the step of forming or full hardening, minute unevenness is formed on the surface of the film. If an intermediate transfer belt made of such a defective film is used to effect secondary transfer, the minute unevenness can cause the generation of minute maltransfer (white mark) and other troubles on the image transferred to the recording medium. On the other hand, the production of a smooth film takes much time to effect evaporation of solvent and hardening of polyamidic acid at the forming and hardening steps, adding to the production cost of belt.
The relationship between the surface resistivity and the volume resistivity of the polyimide resin film having carbon black dispersed therein produced by the foregoing forming process is shown in FIG. 11. As shown in FIG. 11, the polyimide resin film exhibits a volume resistivity of 10.sup.9.5 .OMEGA.cm when the surface resistivity thereof is 10.sup.13 .OMEGA..quadrature..
If the surface resistivity of the intermediate transfer belt exceeds 10.sup.13 .OMEGA./.quadrature., peeling discharge occurs at the post nip portion on the primary transfer portion where the image carrier and the intermediate transfer medium are separated from each other, causing white mark on the discharged portion. Accordingly, in order to avoid the occurrence of white mark with the foregoing intermediate transfer belt composed of a single resin film layer, it is necessary that the allowable volume resistivity fall below 10.sup.9.5 .OMEGA.cm. In this case, the intermediate transfer belt cannot exert an electrostatic force high enough to maintain electric charge for the unfixed toner image transferred to the transfer belt from the image carrier due to its own electric conductivity. Thus, due to mutual electrostatic repulsion force of toner particles or fringe electric field in the vicinity of image edge, the toner flies to the periphery of the image (blur), causing the formation of an image with much noise.
As shown in FIG. 12, which illustrates the relationship between the surface resistivity and the volume resistivity of a polyimide resin film having an electrically-conductive metal oxide dispersed therein, the resin film exhibits a volume resistivity of 10.sup.7.3 .OMEGA.cm when the surface resistivity thereof is 10.sup.13 .OMEGA./.quadrature.. Accordingly, if a metal oxide is used as an electrically conducting agent, there is no range of volume resistivity of resin film where the occurrence of the foregoing white mark and blue can be avoided at the same time.
Since a polyimide resin exhibits excellent mechanical properties, an intermediate transfer belt made of a polyimide resin deforms little when pressed against the image carrier by the bias roll. When a toner image is electrostatically transferred to such an intermediate transfer belt under the action of electric field, the load of pressure by the bias roll is concentrated at the primary transfer site. As a result, the toner image condenses to enhance the charge density, causing the occurrence of discharge inside the toner layer and hence the change of the toner polarity. This phenomenon can cause the occurrence of hollow character, i.e., image defect in which the hollow of line image is blank. This image defect can also occur at the secondary transfer site where the intermediate transfer belt is pressed against the backup roll with a paper provided interposed therebetween by the bias roll.
As a countermeasure against the foregoing image defect there may be proposed a belt material the surface layer of which is made of an elastic material. However, this countermeasure is disadvantageous in that if a rubber material such as silicone rubber is used as a surface material, the toner image cannot be transferred to the recording medium during the secondary transfer due to the adhesivity of the rubber material.
As a countermeasure against image defects such as hollow character, the inventors previously applied for patent an intermediate transfer belt made of a three-layer belt material consisting of a substrate having excellent mechanical properties, an interlayer composed of an elastic material such as fluororubber and a surface layer composed of a material having a small surface energy such as fluororesin, said belt material comprising an electrically conducting agent dispersed only in the substrate (Japanese Patent Application No. 8-236011). However, if the elastic material exhibits a volume resistivity of higher than 10.sup.14 .OMEGA.cm, the surface of the intermediate transfer belt is charged under an electric field developed by the primary transfer, requiring a destaticizing mechanism.
An electrically-conductive plastic belt comprising as a surface layer an electrically-conductive material obtained by incorporating an electrically-conductive filler in a fluororesin in such a proper proportion that the volume resistivity thereof reaches a range of from 10.sup.7 to 10.sup.10 .OMEGA.cm is proposed in JP-A-7-92825. However, the belt disclosed in the above citation is made of a substantially single-layer resin material and thus has no elasticity on the surface resin layer. Therefore, the belt can cause hollow character, i.e., image defect in which the hollow of line image is blank. Further, if the volume resistivity of the belt is lower than 10.sup.9.5 .OMEGA.cm, the electric charge given by a primary transferring apparatus such as bias roll and corotron is removed due to the electrical conductivity of the intermediate transfer medium during the primary transfer of the toner image from the image carrier to the intermediate transfer medium. As a result, blur occurs, causing the formation of an image with much noise as mentioned above. In particular, this phenomenon occurs remarkably in the periphery of an image having a great amount of toner per unit area such as multiple transfer image. This defect can be fatal to color image forming apparatus.
As mentioned above, the prior art intermediate transfer belt material has the following disadvantages. In other words, an electrically-conductive belt material made of a thermoplastic resin having poor mechanical properties deforms greatly when stressed during driving, making it impossible to stably obtain a high quality transfer image. Further, a single-layer belt material made of an electrically-conductive polyimide resin or fluororesin is disadvantageous in that it exhibits too low an allowable range of volume resistivity, causing the occurrence of blur. Moreover, an intermediate transfer belt comprising an elastic layer having no electrically conducting agent dispersed therein is disadvantageous in that it exhibits too high a volume resistivity, requiring a destaticizing mechanism.
On the other hand, a belt material made of a polyimide resin having excellent mechanical properties is disadvantageous in that it deforms little when pressed at the transfer zone under the pressure of the bias roll, causing the toner image to condense and hence generate image defects such as hollow character. Further, a belt material coated with a rubber material such as silicone rubber on the surface thereof is disadvantageous in that the toner image cannot be transferred to the recording medium during the secondary transfer due to the adhesivity of the rubber material.